Point-to-Point Protocol (PPP) is a data link layer protocol for providing encapsulation and transmission of network layer packets on a point-to-point link, wherein the PPP is of a hierarchical structure (physical layer, link layer, network layer); on the underlying layer (the physical layer), it can use a synchronous medium (for example, an Integrated Services Digital Network (ISDN)/H or a synchronous Direct Dial Number (DDN) special line), as well as an asynchronous medium (for example, a Public Switched Telephone Network (PSTN)/a Gateway GPRS Support Node (GGSN) network based on Modem dial).
Since there are many types of physical mediums in the PPP protocol, it is necessary to configure corresponding interfaces for different physical mediums if data forwarding is to be enabled between different physical mediums, it will consequently result in many types of interfaces of central-office devices, complex networking environment and increase of operation cost.